1. Field of the Invention
The present invention relates to the field of the digital signal processing, and more specifically, to the field of plesiochronous synchronization of at least two independently synchronous networks.
2. Discussion of the Prior Art
Timekeeping requires not only accurate frequency control but also accurate time transfer from some master source. Therefore, timekeeping involves comparing two or more clocks to ensure that their time difference is kept within predefined limits over a specified interval. Such a system either can provide relative (synchronized) or absolute timekeeping.
In a synchronized system, each clock is in step with every other clock in the system, but there is no requirement that these clocks to be synchronized to some master clock outside the system. The absolute system, on the other hand, keeps each clock in step with every other clock in the system and also with some master clock outside the system. In large telecommunications networks, the absolute system is used and synchronization is kept with an internationally accepted time scale, such as universal coordinated time (UTC). Smaller networks that have no external time reference use a simple synchronized system.
Notwithstanding the desirability of overall network synchronization, timing islands, or isolated networks, exist as a rule. It means that not all digital telecommunication systems are tied to a large network or even the public network, and not all networks are tied together. This latter situation is especially apparent when international networks are taken into account. Indeed, each country wants to control the timing functions of its national network, so that the master-slave relationship do not exist across international boundaries. The inter-working of such networks requires a standardized and tightly controlled approach to inter-network synchronization. National networks are normally internally synchronized, and when connected to other synchronous networks across international boundaries, are plesiochronous.
The term plesiochronous is derived from the Greek plesio, meaning near, and chronos, time, and refers to the fact that plesiochronous systems run in a state where different parts of the system are almost, but not quite perfectly, synchronized. According to ITU-T, that stands for the Telecom Standardization Organization, previously known as CCITT, of the International Telecommunication Union (ITU), corresponding signals are plesiochronous if their significant instants occur at nominally the same rate, with any variation in rate being constrained within specified limits. In general, plesiochronous systems behave similarly to synchronous systems, except that they must have some means to cope with “sync slips”, which will happen at intervals due to the plesiochronous nature of the system.
The most common example of a plesiochronous system design is the Plesiochronous Digital Hierarchy networking standard. The modern tendency in systems engineering is towards using systems that are either fundamentally asynchronous, such as Ethernet, or fundamentally synchronous, such as Synchronous Digital Hierarchy (SDH), and layering these where necessary, rather than using a mixture between the two in a single technology.
However, if two or more independently synchronized digital networks are not connected, it is a difficult task to make those digital networks plesiochronous. One way to solve this problem is to use the burst communication signals between those digital networks to make them plesiochronous.
Thus, what is needed is to design a technique that utilizes burst communication signals among independently synchronized digital networks to make them plesiochronous.